Micro device with stabilization post

ABSTRACT

A method and structure for stabilizing an array of micro devices is disclosed. A stabilization layer includes an array of stabilization cavities and array of stabilization posts. Each stabilization cavity includes sidewalls surrounding a stabilization post. The array of micro devices is on the array of stabilization posts. Each micro device in the array of micro devices includes a bottom surface that is wider than a corresponding stabilization post directly underneath the bottom surface.

BACKGROUND

1. Field

The present invention relates to micro devices. More particularlyembodiments of the present invention relate to the stabilization ofmicro devices on a carrier substrate.

2. Background Information

Commercial manufacturing and packaging of micro devices often becomesmore challenging as the scale of the micro devices decreases. Someexamples of micro devices include radio frequency (RF)microelectromechanical systems (MEMS) microswitches, light-emittingdiode (LED) display systems, and MEMS or quartz-based oscillators.

One implementation for transferring devices includes peeling devicesfrom an adhesive sheet using a vacuum nozzle that is included in amounting head. Once the device is picked up by the vacuum pressure, itcan be moved by the mounting head to a receiving substrate. A camera mayimage the receiving substrate to assist the system in placing the deviceon the receiving substrate. When the mounting head is positioned in thedesired location above the receiving substrate, the vacuum pressure canbe adjusted to allow the device to stay positioned on the receivingsubstrate while the mounting head moves away from the receivingsubstrate.

In another implementation, devices are formed on an adhesive layer thatis partially removed using a solvent. This results in only a bridgingportion of the adhesive layer connecting the device to a host substrate.To prepare the devices for removal from the substrate, a patternedelastomeric transfer stamp can be selectively applied in order tofracture the bridging portions of the adhesive layer and transfer thedevices from the host substrate.

SUMMARY OF THE INVENTION

A structure and method of forming an array of micro devices which arepoised for pick up are disclosed. In an embodiment, a structure includesa stabilization layer including an array of stabilization posts, and thestabilization layer is formed of a thermoset material such as epoxy orbenzocyclobutene (BCB) which is associated with 10% or less volumeshrinkage during curing, or more particularly about 6% or less volumeshrinkage during curing. An array of micro devices are on the array ofstabilization posts. Each micro device may include a bottom surface thatis wider than a corresponding stabilization post directly underneath thebottom surface. An array of bottom conductive contacts may be formed onthe bottom surfaces of the array micro devices. An array of topconductive contacts may be formed on top of the array of micro devices.In an embodiment the array of stabilization posts are separated by apitch of 1 μm to 100 μm, or more specifically 1 μm to 10 μm.

The stabilization layer may be bonded to a carrier substrate. Thestabilization layer may have an array of stabilization cavities havingstabilization cavity sidewalls surrounding the stabilization posts. Anadhesion promoter layer may be formed between the carrier substrate andthe stabilization layer to increase adhesion. A sacrificial layer mayalso be located between the stabilization layer and the array of microdevices, where the array of stabilization posts also extend through athickness of the sacrificial layer. In an embodiment, the sacrificiallayer is formed of a material such as an oxide or nitride. An adhesionpromoter layer may also be formed between the stabilization layer andthe sacrificial layer to increase adhesion, where the array ofstabilization posts also extend through a thickness of the adhesionpromoter layer. Each stabilization post may be x-y centered below acorresponding micro device or may be off-centered with respect to thecorresponding micro devices.

The array of micro devices may be micro LED devices, and may be designedto emit a specific wavelength such as a red, green, or blue light. In anembodiment, each micro LED device includes a device layer formed of ap-doped semiconductor layer, one or more quantum well layers over thep-doped semiconductor layer, and an n-doped semiconductor layer. Forexample, where the micro LED device is designed to emit a green or bluelight, the p-doped layer may comprise GaN and the n-doped layer may alsocomprise GaN.

One embodiment includes patterning a device layer to form an array ofmicro device mesa structures over a handle substrate, forming apatterned sacrificial layer including an array of openings over thecorresponding array of micro device mesa structures, forming astabilization layer over the patterned sacrificial layer and within thearray of openings, and removing the handle substrate. The stabilizationlayer may be bonded to a carrier substrate prior to removing the handlesubstrate. Bonding the stabilization layer to the carrier substrate mayinclude curing. The stabilization layer may be formed of a thermosetmaterial, which may be BCB in one embodiment.

In an embodiment, the array of openings are formed directly over anarray of conductive contacts of the corresponding array of micro devicemesa structures. In an embodiment, patterning the device layer to formthe array of micro device mesa structures leaves unremoved portions ofthe device layer between the array of micro device mesa structures, andthe unremoved portions of the device layer are subsequently removed toform laterally separate micro LED devices. Removing the unremovedportions of the device layer may include thinning the array of microdevice mesa structures so that an exposed top surface of the array ofmicro LED devices is below an exposed top surface of the patternedsacrificial layer between the micro LED devices. The patternedsacrificial layer is removed to form an open space below and around eachmicro device, in an embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional side view illustration of a bulk LEDsubstrate in accordance with an embodiment of the invention.

FIG. 1B is a cross-sectional side view illustration of a device waferincluding circuitry in accordance with an embodiment of the invention.

FIG. 2A is a cross-sectional side view illustration of a patternedconductive contact layer on a bulk LED substrate in accordance with anembodiment of the invention.

FIG. 2B is a cross-sectional side view illustration of a patternedconductive contact layer on a bulk LED substrate in accordance with anembodiment of the invention.

FIG. 3 is a cross-sectional side view illustration of a device layerpatterned to form an array of micro device mesa structures over a handlesubstrate in accordance with an embodiment of the invention.

FIG. 4 is a cross-sectional side view illustration of an adhesionpromoter layer and a sacrificial layer including an array of openingsformed over an array of micro device mesa structures in accordance withan embodiment of the invention.

FIG. 5 is a cross-sectional side view illustration of a stabilizationlayer formed over an adhesion promoter layer and a sacrificial layer andwithin an array of openings included in the sacrificial layer inaccordance with an embodiment of the invention.

FIG. 6 is a cross-sectional side view illustration of bringing togethera carrier substrate and micro device mesa structures formed on a handlesubstrate in accordance with an embodiment of the invention.

FIG. 7 is a cross-sectional side view illustration of the removal of agrowth substrate in accordance with an embodiment of the invention.

FIG. 8 is a cross-sectional side view illustration of the removal of anepitaxial growth layer and a portion of a device layer in accordancewith an embodiment of the invention.

FIGS. 9A-9B are cross-sectional side view illustrations of patternedconductive contacts formed over an array of laterally separate microdevices in accordance with an embodiment of the invention.

FIG. 10A is a cross-sectional side view illustration of an array ofmicro devices formed on array of stabilization posts after removal of asacrificial layer in accordance with an embodiment of the invention.

FIGS. 10B-10C are schematic top view illustrations of examplestabilization post locations relative to a group of micro devices inaccordance with an embodiment of the invention.

FIGS. 11A-11E are cross-sectional side view illustrations of an array ofelectrostatic transfer heads transferring micro devices from a carriersubstrate to a receiving substrate in accordance with an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe a method and structure forstabilizing an array of micro devices such as micro light emitting diode(LED) devices and micro chips on a carrier substrate so that they arepoised for pick up and transfer to a receiving substrate. For example,the receiving substrate may be, but is not limited to, a displaysubstrate, a lighting substrate, a substrate with functional devicessuch as transistors or integrated circuits (ICs), or a substrate withmetal redistribution lines. While embodiments of some of the presentinvention are described with specific regard to micro LED devicescomprising p-n diodes, it is to be appreciated that embodiments of theinvention are not so limited and that certain embodiments may also beapplicable to other micro semiconductor devices which are designed insuch a way so as to perform in a controlled fashion a predeterminedelectronic function (e.g. diode, transistor, integrated circuit) orphotonic function (LED, laser). Other embodiments of the presentinvention are described with specific regard to micro devices includingcircuitry. For example, the micro devices may be based on silicon or SOIwafers for logic or memory applications, or based on GaAs wafers for RFcommunications applications.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in one embodiment,” “an embodiment”or the like in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “over”, “spanning”, “to”, “between”, and “on” as used hereinmay refer to a relative position of one layer with respect to otherlayers. One layer “over”, “spanning”, or “on” another layer or bonded“to” another layer may be directly in contact with the other layer ormay have one or more intervening layers. One layer “between” layers maybe directly in contact with the layers or may have one or moreintervening layers.

The terms “micro” device, “micro” LED device, or “micro” chip as usedherein may refer to the descriptive size of certain devices, devices, orstructures in accordance with embodiments of the invention. As usedherein the term “micro device” specifically includes “micro LED device”and “micro chip”. As used herein, the terms “micro” devices orstructures are meant to refer to the scale of 1 to 100 μm. However, itis to be appreciated that embodiments of the present invention are notnecessarily so limited, and that certain aspects of the embodiments maybe applicable to larger, and possibly smaller size scales. In anembodiment, a single micro device in an array of micro devices, and asingle electrostatic transfer head in an array of electrostatic transferheads both have a maximum dimension, for example length or width, of 1to 100 μm. In an embodiment, the top contact surface of each microdevice or electrostatic transfer head has a maximum dimension of 1 to100 μm, or more specifically 3 to 20 μm. In an embodiment, a pitch of anarray of micro devices, and a corresponding array of electrostatictransfer heads is (1 to 100 μm) by (1 to 100 μm), for example a 20 μm by20 μm pitch or 5 μm by 5 μm pitch.

In the following embodiments, the mass transfer of an array ofpre-fabricated micro devices with an array of transfer heads isdescribed. For example, the pre-fabricated micro devices may have aspecific functionality such as, but not limited to, an LED forlight-emission, silicon IC for logic and memory, and gallium arsenide(GaAs) circuits for radio frequency (RF) communications. In someembodiments, arrays of micro LED devices which are poised for pick upare described as having a 20 μm by 20 μm pitch, or 5 μm by 5 μm pitch.At these densities, a 6 inch substrate, for example, can accommodateapproximately 165 million micro LED devices with a 10 μm by 10 μm pitch,or approximately 660 million micro LED devices with a 5 μm by 5 μmpitch. A transfer tool including an array of transfer heads matching aninteger multiple of the pitch of the corresponding array of micro LEDdevices can be used to pick up and transfer the array of micro LEDdevices to a receiving substrate. In this manner, it is possible tointegrate and assemble micro LED devices into heterogeneously integratedsystems, including substrates of any size ranging from micro displays tolarge area displays, and at high transfer rates. For example, a 1 cm by1 cm array of micro device transfer heads can pick up and transfer morethan 100,000 micro devices, with larger arrays of micro device transferheads being capable of transferring more micro devices.

In one aspect, embodiments of the invention describe a structure forstabilizing an array of micro devices such as micro light emitting diode(LED) devices on a carrier substrate so that they are poised for pick upand transfer to a receiving substrate. In an embodiment, an array ofmicro devices are held in place on an array of stabilization posts on acarrier substrate. In an embodiment, the stabilization posts are formedof an adhesive bonding material. In this manner, the array ofstabilization posts may retain the array of micro devices in place on acarrier substrate while also providing a structure from which the arrayof micro devices are readily picked up. In an embodiment, the adhesivebonding material includes a thermoset material such as, but not limitedto, benzocyclobutene (BCB) or epoxy. In an embodiment, the thermosetmaterial may be associated with 10% or less volume shrinkage duringcuring, or more particularly about 6% or less volume shrinkage duringcuring. In this manner low volume shrinkage during curing of theadhesive bonding material may not cause delamination between the arrayof stabilization posts and the array of micro devices, and may allow foruniform adhesion between the array stabilization posts and the array ofmicro devices supported by the array of stabilization posts. In oneaspect, the adhesive bonding material (e.g. BCB) forms stabilizationcavity sidewalls that are advantageously positioned to contain microdevice within a stabilization cavity formed by the sidewall. In thisrespect, even if a micro device loses adherence to a stabilization post,it may still be poised for pick up because it is still positioned withinan acceptable tolerance (defined by the stabilization cavity) to betransferred to a receiving substrate.

In one aspect of embodiments of the invention, the array of microdevices are formed in a one sided process sequence in which a devicelayer is etched to form an array of micro device mesa structures priorto applying a stabilization layer (the stabilization layer having theadhesive bonding material that forms the stabilization posts). A onesided process sequence is distinguishable from a two sided processsequence, which is characterized by etching mesa structures in thedevice layer after bonding to a stabilization layer. Suitability of aone sided process or two sided process may depend upon the systemrequirement, and materials being used. For example, where the microdevices are micro LED devices, the devices layers may be formed fromdifferent materials selected for different emission spectra. By way ofexample, a red-emitting micro LED device may be formed of a GaP (5.45 Ålattice constant) based material grown on a GaAs substrate (5.65 Ålattice constant). By way of comparison, a blue-emitting orgreen-emitting micro LED device may be formed of a GaN (5.18 Å latticeconstant) based material grown on a sapphire substrate (4.76 Å latticeconstant). It has been observed that when fabricating devices at the“micro” scale in accordance with the embodiments of the invention, thatthe stored energy in the form of stress in the device layer, typicallygrown by heterogenous growth techniques on a lattice-mismatched growthsubstrate, that stressed device layer may shift upon removal of thegrowth substrate, potentially causing misalignment between the array ofmicro devices that are formed over the array of stabilization posts. Inaccordance with embodiments of the invention, and in particular whenlattice mismatch between the device layer and growth substrate isgreater than 0.2 Å, a one sided process sequence is performed in orderto reduce the amount of shifting between the micro devices andstabilization posts by forming micro device mesa structures onstabilization posts prior to removing the growth substrate.

Without being limited to a particular theory, embodiments of theinvention utilize transfer heads and head arrays which operate inaccordance with principles of electrostatic grippers, using theattraction of opposite charges to pick up micro devices. In accordancewith embodiments of the present invention, a pull-in voltage is appliedto a transfer head in order to generate a grip pressure on a microdevice and pick up the micro device. In accordance with embodiments ofthe invention, the minimum amount pick up pressure required to pick up amicro device from a stabilization post can be determined by the adhesionstrength between the adhesive bonding material from which thestabilization posts are formed and the micro device (or any intermediatelayer), as well as the contact area between the top surface of thestabilization post and the micro device. For example, adhesion strengthwhich must be overcome to pick up a micro device is related to theminimum pick up pressure generated by a transfer head as provided inequation (1):P₂A₁=P₂A₂  (1)

where P₁ is the minimum grip pressure required to be generated by atransfer head, A₁ is the contact area between a transfer head contactsurface and micro device contact surface, A₂ is the contact area on atop surface of a stabilization post, and P₂ is the adhesion strength onthe top surface of a stabilization post. In an embodiment, a grippressure of greater than 1 atmosphere is generated by a transfer head.For example, each transfer head may generate a grip pressure of 2atmospheres or greater, or even 20 atmospheres or greater withoutshorting due to dielectric breakdown of the transfer heads. Due to thesmaller area, a higher pressure is realized at the top surface of thecorresponding stabilization post than the grip pressure generate by atransfer head. In an embodiment, a bonding layer is placed between eachmicro device and stabilization post in order to aid in bonding eachmicro device to a receiving substrate. A variety of different bondinglayers with different melting temperatures are compatible withembodiments of the invention. For example, heat may or may not beapplied to the transfer head assembly, carrier substrate, and/orreceiving substrate during the pick up, transfer, and bondingoperations. In some embodiments, the bonding layer may be acomparatively higher melting temperature material such as gold. In someembodiments the bonding layer is a comparatively lower meltingtemperature material such as indium. In some embodiments, the transferhead assembly may be maintained at an elevated temperature during thepick up and transfer operations in order to assist bonding to thereceiving substrate without thermal cycling of the transfer headassembly. In one embodiment, the bonding layer is gold, and the bondinglayer is not liquefied during the pick up or transfer operations. In oneembodiment the bonding layer is indium, and the bonding layer isliquefied during the pick up and transfer operations. In such anembodiment, the bonding layer may be partially picked up and transferredto the receiving substrate.

In another embodiment, the bonding layer is formed of a materialcharacterized by a low tensile strength. For example, indium ischaracterized by a tensile strength of approximately 4 MPa which can beless than or near the adhesion strength between a gold/BCB bondinginterface of 10 MPa or less, and which is significantly lower than anexemplary 30 MPa adhesion strength between a gold/BCB bonding interface(determined with stud pull test) when treated with adhesion promoterAP3000, an organosilane compound in 1-methoxy-2-propoanol available fromThe Dow Chemical Company. In an embodiment, the bonding layer is cleavedduring the pick up operation due to the lower tensile strength, and aphase change is not created curing the pick up operation. Though, aphase change may still be created in the portion of the bonding layerwhich is picked up with the micro device during placement of the microdevice onto a receiving substrate to aid in bonding of the micro deviceto the receiving substrate.

In another aspect, embodiments of the invention describe a manner offorming an array of micro devices which are poised for pick up in whichconductive contact layers can be formed on top and bottom surfaces ofthe micro devices, and annealed to provide ohmic contacts. Where aconductive contact is formed on a top surface of a micro device, thestabilization layer is formed of a material which is capable ofwithstanding the associated deposition and annealing temperatures. Forexample, a conductive contact may require annealing at temperaturesbetween 200° C. to 350° C. to form an ohmic contact with the microdevice. In this manner, embodiments of the invention may be utilized toform arrays of micro LED devices based upon a variety of differentsemiconductor compositions for emitting various different visiblewavelengths. For example, micro LED growth substrates including activedevices layers formed of different materials for emitting differentwavelengths (e.g. red, green, and blue wavelengths) can all be processedwithin the general sequence of operations of the embodiments.

In the following description exemplary processing sequences aredescribed for forming an array of micro devices on an array ofstabilization posts. Specifically, exemplary processing sequences aredescribed for forming an array of micro LED devices and an array ofmicro chips. Where possible, similar features are illustrated withsimilar annotations in the figures and following description.

FIG. 1A is an example cross-sectional side view illustration of a bulkLED substrate 100 in accordance with an embodiment of the invention. Inthe illustrated embodiment, bulk LED substrate 100 includes a growthsubstrate 102, an epitaxial growth layer 103, and a device layer 105. Inan embodiment, growth substrate 102 is sapphire and may be approximately500 μm thick. Using a sapphire growth substrate may correspond withmanufacturing blue emitting LED devices (e.g. 450-495 nm wavelength) orgreen emitting LED devices (e.g. 495-570 nm wavelength). It is to beappreciated, that while the specific embodiments illustrated anddescribed in the following description may be directed to formation ofgreen or blue emitting LED devices, the following sequences anddescriptions are also applicable to the formation of LED devices thatemit wavelengths other than blue and green. Epitaxial growth layer 103may be grown on growth substrate 102 using known epitaxial growthtechniques. Epitaxial growth layer 103 may be grown on growth substrate102 at a relatively high temperature to facilitate gliding outdislocations in the layer. In an embodiment, epitaxial growth layer 103is a gallium nitride (GaN) based material.

Device layer 105 may be formed on epitaxial growth layer 103, as shownin FIG. 1A. In an embodiment the growth substrate 102 is a sapphiresubstrate and is approximately 200 μm thick. The epitaxial growth layer103 may be any suitable thickness such as between 300 Å-5 μm. In theillustrated embodiment, device layer 105 includes layers for forming LEDdevices. In FIG. 1A, a zoomed-in view of an example device layer 105illustrates one or more quantum well layers 110 between dopedsemiconductor layer 108 (e.g. n-doped) and doped semiconductor layer 112(e.g. p-doped), although the doping of layers 108, 112 may be reversed.In an embodiment, doped semiconductor layer 108 is formed of GaN and isapproximately 0.1 μm to 3 μm thick. The one or more quantum well layers110 may have a thickness of approximately 0.5 μm. In an embodiment,doped semiconductor layer 112 is formed of GaN, and is approximately 0.1μm to 2 μm thick.

FIG. 1B is a cross-sectional side view illustration of a device waferincluding circuitry in accordance with an embodiment of the invention.In accordance with embodiments of the invention, the device wafer 180may be formed of a variety of materials depending upon the desiredfunction. For example, in an embodiment, the device wafer 180 is asilicon wafer, or silicon-on-insulator (SOI) wafer for logic or memory.In an embodiment, the device wafer 180 is a gallium arsenide (GaAs)wafer for radio frequency (RF) communications. These are merelyexamples, and embodiments of the invention envision are not limited tosilicon or GaAs wafers, nor are embodiments limited to logic, memory, orRF communications.

In an embodiment, the device wafer 180 includes an active device layer185, optional buried oxide layer 184, and base substrate 182. In theinterest of clarity, the following description is made with regard to anSOI device wafer 180, including an active device layer 185, buried oxidelayer 184, and base silicon substrate 182, though other types of deviceswafers may be used, including bulk semiconductor wafers. In anembodiment, the active device layer 185 may include working circuitry tocontrol one or more LED devices. In some embodiments, back-endprocessing may be performed within the active device layer 185.Accordingly, in an embodiment, the active device layer 185 includes anactive silicon layer 187 including a device such as a transistor, metalbuild-up layers 188 including interconnects 189, bonding pads 190, andpassivation 192.

In interests of clarity the remainder of the description is made withregard to the bulk LED substrate of FIG. 1A. However, it is appreciatedthat the process sequences in the following description may be used tofabricate other micro devices. For example, micro chips may be similarlymanufactured by substituting bulk LED substrate 100 with device wafer180 and using the same or similar processes as described with referenceto bulk LED substrate 100. Accordingly, in the following description,both the growth substrate 102 and base substrate 182 can alternativelybe referred to more generically as a “handle” substrate so as to notpreclude the processing sequence on a growth substrate 102 from beingapplied to the a processing sequence on a base substrate 182.

FIG. 2A is a cross-sectional side view illustration of a patternedconductive contact layer on bulk LED substrate 100 in accordance with anembodiment of the invention. A conductive contact layer may be formedover device layer 105 using a suitable technique such as sputtering orelectron beam physical deposition followed by etching or liftoff to formthe array of conductive contacts 120. In an embodiment, the array ofconductive contacts 120 have a thickness of approximately 0.1 μm-2 μm,and may include a plurality of different layers. For example, aconductive contact 120 may include an electrode layer 121 for ohmiccontact, a mirror layer 122, an adhesion/barrier layer 123, a diffusionbarrier layer 124, and a bonding layer 125. In an embodiment, electrodelayer 121 may make ohmic contact to a p-doped semiconductor layer 112,and may be formed of a high work-function metal such as nickel. In anembodiment, a minor layer 122 such as silver is formed over theelectrode layer 121 to reflect the transmission of the visiblewavelength. In an embodiment, titanium is used as an adhesion/barrierlayer 123, and platinum is used as a diffusion barrier 124 to bondinglayer 125. Bonding layer 125 may be formed of a variety of materialswhich can be chosen for bonding to the receiving substrate and/or toachieve the requisite tensile strength or adhesion or surface tensionwith the stabilization posts. Following the formation of layers 121-125,the substrate stack can be annealed to form an ohmic contact. Forexample, a p-side ohmic contact may be formed by annealing the substratestack at 510° C. for 10 minutes.

In an embodiment, bonding layer 125 is formed of a conductive material(both pure metals and alloys) which can diffuse with a metal forming acontact pad on a receiving substrate (e.g. gold, indium, or tin contactpad) and has a liquidus temperature above 200° C. such as tin (231.9°C.) or bismuth (271.4° C.), or a liquidus temperature above 300° C. suchas gold (1064° C.) or silver (962° C.). In some embodiments, bondinglayer 125 such as gold may be selected for its poor adhesion with theadhesive bonding material used to form the stabilization posts. Forexample, noble metals such as gold are known to achieve poor adhesionwith BCB. In this manner, sufficient adhesion is created to maintain thearray of micro LED devices on the stabilization posts during processingand handling, as well as to maintain adjacent micro LED devices in placewhen another micro LED device is being picked up, yet also not createtoo much adhesion so that pick up can be achieved with an applied pickup pressure on the transfer head of 20 atmospheres or less, or moreparticularly 5-10 atmospheres.

In the embodiment illustrated in FIG. 2A, where bonding layer 125 has aliquidus temperature above the annealing temperature for forming thep-side ohmic contact, the anneal (e.g. 510° C. for 10 minutes) can beperformed after the formation of the patterned conductive contact layer120, including bonding layer 125. Where bonding layer 125 has a liquidustemperature below the annealing temperature for forming the p-side ohmiccontact, the bonding layer 125 may be formed after annealing.

FIG. 2B is a cross-sectional side view illustration of a patternedconductive contact layer on a bulk LED substrate 100 in accordance withan embodiment of the invention. The embodiment illustrated in FIG. 2Bmay be particularly useful where bonding layer 125 is formed of amaterial with a liquidus temperature below the annealing temperature ofthe p-side ohmic contact, though the embodiment illustrated in FIG. 2Bis not limited to such and may be used where the bonding layer 125 isformed of a material with a liquidus temperature above the annealingtemperature of the p-side ohmic contact. In such embodiments, electrodelayer 121 and minor layer 122 may be formed similarly as described withregard to FIG. 2A. Likewise, adhesion/barrier layer 123 and diffusionbarrier 124 may be formed similarly as described with regard to FIG. 2Awith one difference being that the layers 123, 124 may optionally wraparound the sidewalls of the layers 121, 122. Following the formation oflayers 121-124, the substrate stack can be annealed to form an ohmiccontact. For example, a p-side ohmic contact may be formed by annealingthe substrate stack at 510° C. for 10 minutes. After annealing layer121-124 to form the p-side ohmic contact, the bonding layer 125 may beformed. In an embodiment, the bonding layer 125 has a smaller width thanfor layers 121-124.

In an embodiment, bonding layer 125 has a liquidus temperature ormelting temperature of approximately 350° C. or lower, or morespecifically of approximately 200° C. or lower. At such temperatures thebonding layer may undergo a phase change without substantially affectingthe other components of the micro LED device. In an embodiment, theresultant bonding layer may be electrically conductive. In accordancewith some embodiments, the bonding layer 125 may be a solder material,such as an indium, bismuth, or tin based solder, including pure metalsand metal alloys. In a particular embodiment, the bonding layer 125 isindium.

FIG. 3 is a cross-sectional side view illustration of device layer 105patterned to form an array of micro device mesa structures 127 over ahandle substrate that includes growth substrate 102 and epitaxial growthlayer 103 in accordance with an embodiment of the invention. Etching oflayers 108, 110, and 112 of device layer 105 may be accomplished usingsuitable etch chemistries for the particular materials. For example,n-doped semiconductor layer 108, quantum well layer(s) 110, and p-dopedlayer 112 may be dry etched in one operation with a BCl₃ and Cl₂chemistry. As FIG. 3 illustrates, device layer 105 may not be etchedcompletely through which leaves unremoved portions 129 of device layer105 that connect the micro device mesa structures 127. In one example,the etching of device layer 105 is stopped in n-doped semiconductorlayer 108 (which may be n-doped GaN). Height of the micro device mesastructures 127 (not including the thickness of the unremoved portions129 may correspond substantially to the height of the laterally separatemicro devices to be formed. In accordance with embodiments of theinvention, the device layer 105 may alternatively be completely etchedthrough. For example, where the bulk LED substrate 100 is replaced witha device wafer 180 in the processing sequence, etching may stop on theburied oxide layer 184.

FIG. 4 is a cross-sectional side view illustration of an adhesionpromoter layer 144 and a sacrificial layer 135 including an array ofopenings 133 formed over the array of micro device mesa structures 127in accordance with an embodiment of the invention. In an embodiment,sacrificial layer 135 is between approximately 0.5 and 2 microns thick.In an embodiment, sacrificial layer 135 is formed of an oxide (e.g.SiO₂) or nitride (e.g. SiN_(x)), though other materials may be usedwhich can be selectively removed with respect to the other layers. In anembodiment, sacrificial layer 135 is deposited by sputtering, lowtemperature plasma enhanced chemical vapor deposition (PECVD), orelectron beam evaporation to create a low quality layer, which may bemore easily removed than a higher quality layer deposited by othermethods such as atomic layer deposition (ALD) or high temperature PECVD.

Still referring to FIG. 4, after the formation of sacrificial layer 135,an adhesion promoter layer 144 may optionally be formed in order toincrease adhesion of the stabilization layer 145 (not yet formed) to thesacrificial layer 135. A thickness of 100-300 angstroms may besufficient to increase adhesion.

Specific metals that have good adhesion to both the sacrificial layer135 and a BCB stabilization layer (not yet formed) include, but are notlimited to, titanium and chromium. For example, sputtered or evaporatedtitanium or chromium can achieve an adhesion strength (stud pull) ofgreater than 40 MPa with BCB.

After forming sacrificial layer 135, the sacrificial layer 135 ispatterned to form an array of openings 133 over the array of conductivecontacts 120. If adhesion layer 144 is present, it can also be patternedto form the array of openings 133, exposing the array of conductivecontacts 120 as illustrated in FIG. 4. In an example embodiment, afluorinated chemistry (e.g. HF vapor, or CF₄ or SF₆ plasma) is used toetch the SiO₂ or SiN_(x) sacrificial layer 135.

As will become more apparent in the following description the height,and length and width of the openings 133 in the sacrificial layer 135correspond to the height, and length and width (area) of thestabilization posts to be formed, and resultantly the adhesion strengththat must be overcome to pick up the array of micro devices (e.g. microLED devices) poised for pick up on the array of stabilization posts. Inan embodiment, openings 133 are formed using lithographic techniques andhave a length and width of approximately 1 μm by 1 μm, though theopenings may be larger or smaller so long as the openings have a width(or area) that is less than the width (or area) of the conductivecontacts 120 and/or micro LED devices. Furthermore, the height, lengthand width of the openings 131 between the sacrificial layer 135 formedalong sidewalls between the micro device mesa structures 127 willcorrespond to the height, length and width of the stabilization cavitysidewalls to be formed. Accordingly, increasing the thickness of thesacrificial layer 135 and space separating adjacent micro device mesastructures 127 will have the effect of decreasing the size of thestabilization cavity sidewalls.

FIG. 5 is a cross-sectional side view illustration of a stabilizationlayer 145 formed over adhesion promoter layer 144 and sacrificial layer135 and within an array of openings 133 included in sacrificial layer135 in accordance with an embodiment of the invention. Stabilizationlayer 145 may be formed of an adhesive bonding material. The adhesivebonding material may be a thermosetting material such asbenzocyclobutene (BCB) or epoxy. In an embodiment, the thermosettingmaterial may be associated with 10% or less volume shrinkage duringcuring, or more particularly about 6% or less volume shrinkage duringcuring so as to not delaminate from the conductive contacts 120 on themicro device mesa structures 127. In order to increase adhesion to theunderlying structure, in addition to, or in alternative to adhesionpromoter layer 144, the underlying structure can be treated with anadhesion promoter such as AP3000, available from The Dow ChemicalCompany, in the case of a BCB stabilization layer in order to conditionthe underlying structure. AP3000, for example, can be spin coated ontothe underlying structure, and soft-baked (e.g. 100° C.) or spun dry toremove the solvents prior to applying the stabilization layer 145 overthe patterned sacrificial layer 135.

In an embodiment, stabilization layer 145 is spin coated or spray coatedover the patterned sacrificial layer 135, though other applicationtechniques may be used. Following application of the stabilization layer145, the stabilization may be pre-baked to remove solvents, resulting ina b-staged layer. In an embodiment, the stabilization layer 145 isthicker than the height of openings 131, when present, between microdevice mesa structures 127, and openings 133 in the patternedsacrificial layer 135. In this manner, the thickness of thestabilization layer filling openings 133 will become stabilization posts152, the thickness of the stabilization layer filling openings 131 willbecome stabilization cavity sidewalls 147, and the remainder of thethickness of the stabilization layer 145 over the filled openings 131,133 can function to adhesively bond the bulk LED substrate 100 to acarrier substrate.

FIG. 6 is a cross-sectional side view illustration of bringing together(bonding) a carrier substrate 160 and micro device mesa structures 127formed on the handle substrate in accordance with an embodiment of theinvention. In order to increase adhesion with the stabilization layer145, an adhesion promoter layer 162 can be applied to the carriersubstrate 160 prior to bonding the bulk LED substrate 100 to the carriersubstrate 160 similarly as described above with regard to adhesionpromoter layer 144. Likewise, in addition to, or in alternative toadhesion promoter layer 162, an adhesion promoter such as AP3000 may beapplied to the surface of the carrier substrate 160 or adhesion promoterlayer 162. Carrier substrate 160 may be silicon, for example.

Alternatively stabilization layer 145 can be formed on carrier substrate160 prior to bonding the carrier substrate 160 to the handle substrate.For example, the structure including the patterned sacrificial layer 135and micro device mesa structures 127 can be embossed into an a-staged orb-staged stabilization layer 145 formed on the carrier substrate 160.

Depending upon the particular material of stabilization layer 145,stabilization layer 145 may be thermally cured, or cured withapplication of UV energy. In an embodiment, stabilization layer 145 isa-staged or b-staged prior to bonding the carrier substrate to thehandle substrate, and is cured at a temperature or temperature profileranging between 150° C. and 300° C. Where stabilization layer 145 isformed of BCB, curing temperatures should not exceed approximately 350°C., which represents the temperature at which BCB begins to degrade. Inaccordance with embodiments including a bonding layer 125 materialcharacterized by a liquidus temperature (e.g. gold, silver, bismuth)greater than 250° C., full-curing of a BCB stabilization layer 145 canbe achieved in approximately 1 hour or less at a curing temperaturebetween 250° C. and 300° C. Other bonding layer 125 materials such as Sn(231.9° C.) may require between 10-100 hours to fully cure attemperatures between 200° C. and the 231.9° C. liquidus temperature. Inaccordance with embodiments including a bonding layer 125 materialcharacterized by a liquidus temperature below 200° C. (e.g. indium), aBCB stabilization layer 145 may only be partially cured (e.g. 70% orgreater). In such an embodiment the BCB stabilization layer 145 may becured at a temperature between 150° C. and the liquidus temperature ofthe bonding layer (e.g. 156.7° C. for indium) for approximately 100hours to achieve at least a 70% cure.

Achieving a 100% full cure of the stabilization layer is not required inaccordance with embodiments of the invention. More specifically, thestabilization layer 145 may be cured to a sufficient curing percentage(e.g. 70% or greater for BCB) at which point the stabilization layer 145will no longer reflow. Moreover, it has been observed that suchpartially cured (e.g. 70% or greater) BCB stabilization layer 145 maypossess sufficient adhesion strengths with the carrier substrate 160 andpatterned sacrificial layer 135 (or any intermediate layer(s)).

FIG. 7 is a cross-sectional side view illustration of the removal ofgrowth substrate 102 in accordance with an embodiment of the invention.When growth substrate 102 is sapphire, laser lift off (LLO) may be usedto remove the sapphire. Removal may be accomplished by other techniquessuch as grinding and etching, depending upon the material selection ofthe growth substrate 102.

FIG. 8 is a cross-sectional side view illustration of the removal ofepitaxial growth layer 103 and a portion of device layer 105 inaccordance with an embodiment of the invention. The removal of epitaxialgrowth layer 103 and a portion of device layer 105 may be accomplishedusing one or more of Chemical-Mechanical-Polishing (CMP), dry polishing,or dry etch. FIG. 8 illustrates that that unremoved portions 129 ofdevice layer 105 that connected the micro device mesa structures 127(FIG. 7) are removed in FIG. 8, which leaves laterally separated microdevices 128. In an embodiment, removing unremoved portions 129 of devicelayer 105 includes thinning the array of micro device mesa structures127 so that an exposed top surface 109 of each of the laterally separatemicro devices 128 are below an exposed top surface 139 of patternedsacrificial layer 135.

In embodiments where the bulk LED substrate 100 includes epitaxialgrowth layer 103, a portion of the doped semiconductor layer 108adjacent the epitaxial growth layer may also function as a “buffer”. Forexample, epitaxial growth layer 103 may or may not be doped, whilesemiconductor layer 108 is n-doped. It may be preferred to remove theepitaxial growth layer 103 using any suitable technique such as wet ordry etching, or chemical mechanical polishing (CMP), followed by a timedetch of the remainder of the doped semiconductor layer 108 resulting inthe structure illustrated in FIG. 8. In this manner, the thickness ofthe laterally separate micro devices 128 is largely determined by theetching operation illustrated in FIG. 3 for the formation of the microdevice mesa structures 127, combined with the timed etch or etch stopdetection of the etching operation illustrated in FIG. 8.

FIGS. 9A-9B are cross-sectional side view illustrations of a patternedconductive contacts 175 formed over an array of laterally separatedmicro device 128 in accordance with an embodiment of the invention.FIGS. 9A and 9B are substantially similar, with a difference being thearrangement of layers within conductive contacts 120. FIG. 9Acorresponds with the conductive contacts 120 illustrated in FIG. 2Awhile FIG. 9B corresponds with the conductive contacts 120 illustratedin FIG. 2B.

To form conductive contacts 175, a conductive contact layer is formedover micro devices 128 and sacrificial layer 135. The conductive contactlayer may be formed of a variety of conductive materials includingmetals, conductive oxides, and conductive polymers. In an embodiment,conductive contacts are formed of a metal or metal alloy. In anembodiment, the conductive contact layer is formed using a suitabletechnique such as sputtering or electron beam physical deposition. Forexample, the conductive contact layer may include BeAu metal alloy, or ametal stack of Au/GeAuNi/Au layers. The conductive contact layer canalso be a combination of one or more metal layers and a conductiveoxide. In an embodiment, after forming the conductive contact layer, thesubstrate stack is annealed to generate an ohmic contact between theconductive contact layer and the device layer of micro devices 128.Where the stabilization layer is formed of BCB, the annealingtemperature may be below approximately 350° C., at which point BCBdegrades. In an embodiment, annealing is performed between 200° C. and350° C., or more particularly at approximately 320° C. for approximately10 minutes. After the conductive contact layer is deposited, it can bepatterned and etched to form conductive contacts 175, which may ben-metal conductive contacts.

The resultant structures illustrated in FIGS. 9A and 9B are robustenough for handling and cleaning operations to prepare the substratestructure for subsequent sacrificial layer removal and electrostaticpick up. In an exemplary embodiment where the array of micro deviceshave a pitch of 5 microns, each micro device may have a minimum width(e.g. along the top surface 109) of 4.5 μm, and a separation betweenadjacent micro devices of 0.5 μm. It is to be appreciated that a pitchof 5 microns is exemplary, and that embodiments of the inventionencompass any pitch of 1 to 100 μm as well as larger, and possiblysmaller pitches.

FIGS. 9A and 9B illustrate a structure having a stabilization layer 145that includes an array of stabilization cavities and an array ofstabilization posts 152. Each stabilization cavity in the array includessidewalls 147 (which may be coated with adhesion promoter layer 144) ofstabilization layer 145 that surround stabilization posts 152. In FIGS.9A and 9B, the bottom surface 107 (having dimension D1) of each microdevice 128 is wider that the corresponding stabilization post 152 thatis directly under the micro device 128. In FIGS. 9A and 9B, sacrificiallayer 135 spans along side surfaces 106 of micro devices 128. In theillustrated embodiments, stabilization posts 152 extend through athickness of sacrificial layer 135 and the stabilization cavitysidewalls 147 of the stabilization layer 145 are taller than thestabilization posts 152. However, in some embodiments, stabilizationposts 152 are taller than the stabilization cavity sidewalls 147. Forexample, the thickness of the sacrificial layer 135 and space betweenlaterally adjacent micro devices 128 may affect the size of thestabilization cavity sidewalls 147.

FIG. 10A is a cross-sectional side view illustration of an array ofmicro devices 128 formed on array of stabilization posts 152 afterremoval of sacrificial layer 135 in accordance with an embodiment of theinvention. In the embodiments illustrated, sacrificial layer 135 isremoved resulting in an open space 177 between each micro device 128 andstabilization layer 145. As illustrated, open space 177 includes theopen space below each micro device 128 and stabilization layer 145 aswell as the open space between each micro device 128 and stabilizationcavity sidewalls 147 of stabilization layer 145. A suitable etchingchemistry such as HF vapor, CF₄, or SF₆ plasma may be used to etch theSiO₂ or SiN_(x) of sacrificial layer 135.

After sacrificial layer 135 is removed, the array of micro devices 128are on the array of stabilization posts 152 are supported only by thearray of stabilization posts 152. At this point, the array of microdevices 128 are poised for pick up transferring to a target or receivingsubstrate. After sacrificial layer 135 is removed leaving onlystabilization posts 152 to support micro devices 128, it is possiblethat a micro device 128 may shift off of its corresponding stabilizationpost 152. However, in the illustrated embodiment, the stabilizationcavity sidewalls 147 may be advantageously positioned to contain theshifted micro device 128 within the stabilization cavity. Therefore,even when a micro device 128 loses adherence to a stabilization post152, it may still be poised for pick up because it is still positionedwithin an acceptable tolerance (defined by the stabilization cavity) tobe transferred to a receiving substrate.

To further illustrate, FIGS. 10B-10C are schematic top viewillustrations of example stabilization post 152 locations relative to agroup of micro devices 128 in accordance with an embodiment of theinvention. The cross-sectional side view of FIG. 10A is illustratedalong line A-A in FIGS. 10B and 10C. FIG. 10B shows an embodiment wherestabilization posts 152 are centered in the x-y directions relative to atop view illustration of micro devices 128. FIG. 10B also shows howstabilization cavity sidewalls 147 can function to contain micro devices128, if a micro device 128 loses adhesion to a stabilization post 152.FIG. 10C is substantially similar to FIG. 10B except that stabilizationposts 153 have replaced stabilization posts 152. Stabilization posts 153differ from stabilization posts 152 in that they are not centered in thex-y direction relative to a top view illustration of the micro devices128. Of course, positions of stabilization posts other than theillustrated positions of stabilization posts 152 and 153 are possible.In an embodiment, during the pick up operation described below theoff-centered stabilization posts 153 may provide for the creation of amoment when the array of transfer heads contact the array of microdevices in which the micro devices tilt slightly as a result of theapplied downward pressure from the array of transfer heads. This slighttilting may aid in overcoming the adhesion strength between thestabilization posts 153 and the array of micro devices 128. Furthermore,such assistance in overcoming the adhesion strength may potentiallyallow for picking up the array of micro devices with a lower grippressure. Consequently, this may allow for operation of the array oftransfer heads at a lower voltage, and impose less stringent dielectricstrength requirements in the dielectric layer covering each transferhead required to achieve the electrostatic grip pressure.

FIGS. 11A-11E are cross-sectional side view illustrations of an array ofelectrostatic transfer heads 204 transferring micro devices 128 from acarrier substrate 160 to a receiving substrate 300 in accordance with anembodiment of the invention. FIG. 11A is a cross-sectional side viewillustration of an array of micro device transfer heads 204 supported bysubstrate 200 and positioned over an array of micro devices 128stabilized on stabilization posts 152 of stabilization layer 145 oncarrier substrate 160. The array of micro devices 128 are then contactedwith the array of transfer heads 204 as illustrated in FIG. 11B. Asillustrated, the pitch of the array of transfer heads 204 is an integermultiple of the pitch of the array of micro devices 128. A voltage isapplied to the array of transfer heads 204. The voltage may be appliedfrom the working circuitry within a transfer head assembly 206 inelectrical connection with the array of transfer heads through vias 207.The array of micro devices 128 is then picked up with the array oftransfer heads 204 as illustrated in FIG. 11C. The array of microdevices 128 is then placed in contact with contact pads 302 (e.g. gold,indium, or tin) on a receiving substrate 300, as illustrated in FIG.11D. The array of micro devices 128 is then released onto contact pads302 on receiving substrate 300 as illustrated in FIG. 11E. For example,the receiving substrate may be, but is not limited to, a displaysubstrate, a lighting substrate, a substrate with functional devicessuch as transistors or ICs, or a substrate with metal redistributionlines.

In accordance with embodiments of the invention heat may be applied tothe carrier substrate, transfer head assembly, or receiving substrateduring the pickup, transfer, and bonding operations. For example, heatcan be applied through the transfer head assembly during the pick up andtransfer operations, in which the heat may or may not liquefy the microdevice bonding layers 125. The transfer head assembly may additionallyapply head during the bonding operation on the receiving substrate thatmay or may not liquefy one of the bonding layers on the micro device orreceiving substrate to cause diffusion between the bonding layers.

The operation of applying the voltage to create a grip pressure on thearray of micro devices can be performed in various orders. For example,the voltage can be applied prior to contacting the array of microdevices with the array of transfer heads, while contacting the microdevices with the array of transfer heads, or after contacting the microdevices with the array of transfer heads. The voltage may also beapplied prior to, while, or after applying heat to the bonding layers.

Where the transfer heads 204 include bipolar electrodes, an alternatingvoltage may be applied across a the pair of electrodes in each transferhead 204 so that at a particular point in time when a negative voltageis applied to one electrode, a positive voltage is applied to the otherelectrode in the pair, and vice versa to create the pickup pressure.Releasing the array of micro devices from the transfer heads 204 may beaccomplished with a varied of methods including turning off the voltagesources, lower the voltage across the pair of silicon electrodes,changing a waveform of the AC voltage, and grounding the voltagesources.

Furthermore, the method of pickup up and transferring the array of microdevices from a carrier substrate to a receiving substrate described withregard to FIGS. 11A-11E is applicable contexts where the micro devicesare micro LEDs or other examples of micro devices described herein.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for stabilizing an array of microdevices on a carrier substrate, and for transferring the array of microdevices. Although the present invention has been described in languagespecific to structural features and/or methodological acts, it is to beunderstood that the invention defined in the appended claims is notnecessarily limited to the specific features or acts described. Thespecific features and acts disclosed are instead to be understood asparticularly graceful implementations of the claimed invention usefulfor illustrating the present invention.

What is claimed is:
 1. A structure comprising: a stabilization layercomprising an array of stabilization cavities and an array ofstabilization posts, wherein each stabilization cavity includes astabilization post and sidewalls that surround and are taller than acorresponding stabilization post; and an array of micro devices on thearray of stabilization posts; wherein each micro device in the array ofmicro devices includes a bottom surface that is wider than acorresponding stabilization post directly underneath the bottom surface.2. The structure of claim 1, further comprising a sacrificial layerbetween the stabilization layer and the array of micro devices, whereinthe array of stabilization posts extend through a thickness of thesacrificial layer, and wherein the sacrificial layer spans along a sidesurface of each of the micro devices in the array of micro devices, theside surface running between a top surface and the bottom surface ofeach micro device in the array.
 3. The structure of claim 2, wherein thesacrificial layer comprises an oxide or nitride.
 4. The structure ofclaim 2, wherein the micro devices are micro LED devices.
 5. Thestructure of claim 1, further comprising an array of bottom conductivecontacts on the bottom surfaces of the array of micro devices.
 6. Thestructure of claim 1, wherein the stabilization layer is formed of athermoset material.
 7. The structure of claim 6, wherein the thermosetmaterial comprises benzocyclobutene (BCB).
 8. The structure of claim 1,wherein the array of stabilization posts are separated by a pitch of 1μm to 10 μm.
 9. A structure comprising: a stabilization layer comprisingan array of stabilization cavities and an array of stabilization posts,wherein each stabilization cavity includes sidewalls that surround acorresponding stabilization post; and an array of micro devices on thearray of stabilization posts; wherein each micro device in the array ofmicro devices includes a bottom surface that is wider than acorresponding stabilization post directly underneath the bottom surface,and each micro device includes an active device layer that is partiallycontained within a corresponding stabilization cavity.
 10. The structureof claim 9, wherein the device layer includes a p-n diode.
 11. Thestructure of claim 10, wherein the device layer includes a transistor.12. The structure of claim 9, wherein the stabilization cavity sidewallsare taller than the stabilization posts.
 13. The structure of claim 9,further comprising a sacrificial layer between the stabilization layerand the array of micro devices, wherein the array of stabilization postsextend through a thickness of the sacrificial layer, and wherein thesacrificial layer spans along a side surface of each of the microdevices in the array of micro devices, the side surface running betweena top surface and the bottom surface of each micro device in the array.14. The structure of claim 13, wherein the sacrificial layer comprisesan oxide or nitride.
 15. The structure of claim 9, further comprising anarray of bottom conductive contacts on the bottom surfaces of the arrayof micro devices.
 16. The structure of claim 9, wherein thestabilization layer is formed of a thermoset material.
 17. The structureof claim 16, wherein the thermoset material comprises benzocyclobutene(BCB).
 18. The structure of claim 9, wherein the array of stabilizationposts are separated by a pitch of 1 μm to 10 μm.